The present invention relates to a method of detecting with the aid of X-ray radiation heterogeneities in joints located between sections of insulation on pipe assemblies in communal or district heating systems and in the insulating material itself, using a radiation sensitive material, such as a film, said pipe assemblies being of the kind which incorporate an inner steel pipe, a tubular plastics jacket encasing the steel pipe and spaced radially therefrom, and a foamed plastics insulation which fills the space between the steel pipe and the jacket therearound. The invention also relates to an apparatus for carrying out the method.
In recent years damage caused by corrosion of the steel pipes of district heating systems has become particularly serious. This corrosion is caused by water penetrating cracks and apertures in the tubular jacket, or in adjacent tubular lengths thereof.
These water entry points are created in many instances by heterogeneities in the insulation of the straight pipes, and in the foam mouldings at the joint locations. One normally found heterogeneity comprises bubble formations in the insulating material, which nomally comprises polyurethane foam, unfilled sections, or open gaps at locations where the insulation at the joints connects with the insulation of the straight pipes. When an air-filled space forms in a joint or in the insulation present between mutually joined sections, an abnormally large quantity of heat is transported from the steel pipe conducting hot water, out to the tubular jacket or to the sealing sleeve embracing a joint, via this unfilled space. Because the hot water has a temperature which reaches 120.degree. C., and because the tubular jacket is made of a plastics material, such as polyethylene, whose plastizing temperature often lies beneath 100.degree. C., this heat transportation will cause the jacket material to soften. The soil-stone filling surrounding the jacket is then able to deform the softened part thereof and rapidly cause the jacket to be punctured, whereupon water enters the insulation and initiates corrosion of the steel pipe. Such heat transportation is particularly critical in those instances where shrink sleeves are used at the joints, since the tensioning force exerted by the sleeves, and therewith their sealing ability, ceases in the presence of high temperatures.
It is therefore of the greatest importance that the foamed plastics insulation at the joint locations is free from air bubbles or unfilled voids capable of creating abnormal heat transfers.
The opportunities of checking the quality of the foamed plastics injected into the joint between two pipe lengths on the working site are very slight and in practice are limited to visual inspections. The possibilities of making such quality checks in the case of the filling in the space between the steel pipe and the tubular jacket, which is normally concentric with the steel pipe, are also very limited. The only possibility of making quality checks at present involves the destructive removal of samples. In addition hereto, it is possible by means of the X-ray method to establish the moisture content of the insulation within certain limits, for example the ends of straight pipes or, for example, at excavated joint locations where water leakage is suspected.
Attempts have been made to examine such insulation with the aid of conventional X-ray techniques. The results, however, have been negative, due to the fact that the steel pipe absorbs so much of the radiation that the radiation is unable to reach parts of the insulation lying therebeneath, and that if an attempt is made to reach the underlying parts of the insulation, by increasing the level of radiation energy transmitted thereto, insufficient contrast is obtained, thereby making it impossible to discover voids and like unfilled parts.